This invention relates to the treatment of minerals, and more particularly is concerned with the treatment of minerals which can be employed in the coating of paper.
Many varieties of coated paper are produced today. They are principally coated with a paper coating composition, sometimes known as the coating colour, which essentially comprises an adhesive, also known as a binder, and a pigment dispersed in a suitable liquid medium. The adhesive used can be, for example, starch, casein or a synthetic resin latex; the particular adhesive used will depend, for example, on the printing process to be used, e.g. offset lithography requires the adhesive to be water-insoluble. Generally, the pigment will consist of clay, as the main constituent, and minor amounts, i.e. less than 20% by weight, of one or more other constituents, for example calcium carbonate, lithopone, barium sulphate, vitanium pigments, talc or satin white. The clays normally used for coating purposes comprise at least 70% by weight of particles having an equivalent spherical diameter less than two microns. The paper coating composition is usually prepared by dispersing the pigment and adhesive separately in the liquid medium, which is usually water, and mixing the two components together. The paper coating composition is then spread over the surface of the paper, dired and subjected to a finishing process, e.g. calendering or glazing. The principal properties of a paper which are affected by coating are its reflectance to light, its smoothness, its ink receptivity and its gloss.
Conventionally, the suitability of the optical properties of a mineral, for example a kaolinitic clay, which is to be used for paper coating purposes, is evaluated by determining the reflectance to visible light of a dry powder of the mineral or of a thick, adhesive-free coating of the mineral on paper. These tests are taken to be indicative of the probable performance of the mineral when used in a paper coating composition but they are not infallible and it often happens that a mineral which gives good results in these tests shows inferior behaviour when incorporated in a paper coating composition containing an adhesive.
It is an object of the present invention to prepare a mineral which is suitable for coating onto paper and which, when coated onto paper in a paper coating composition comprising an adhesive, provides a coated paper sheet of good reflectance to light.